The present invention relates to the welding of copper armature conductors to the commutator during the production of automotive starting motors.
Automotive starting motors are typically DC machines including a field winding on the stator, an armature winding on the rotor and a mechanical rectifier known as a commutator. The stator comprises a laminated ferromagnetic material equipped with protrusions around which the coils of the field winding are wrapped. The rotor includes a laminated core which is slotted to accommodate the armature winding. The armature winding is comprised of a plurality of copper armature conductors wound on the slots of the rotor. The commutator is a mechanical rectifier comprised of a plurality of parallel copper segments insulated from one another and arranged in cylindrical fashion. Carbon brushes ride on the commutator and serve to conduct direct current to the armature winding.
In production of the automotive starting motor, the copper armature conductors must be joined to the copper segments of the commutator to provide a connection between the armature winding and the commutator. The copper armature conductors are typically joined to the commutator using a process of welding commonly referred to as xe2x80x9chot staking.xe2x80x9d An armature 10 and hot staking machine 20 are represented in FIG. 1. Hot staking of the armature conductors 11 to the commutator 12 involves application of the tungsten electrode 22 of the hot staking machine 20 to a particular set of armature conductors 11. After the electrode 12 contacts the conductors 11, the electrode 12 is heated by passing electric current through the electrode. The electrode also applies downward force to the conductors 11 using the air cylinder 24.
FIG. 2 shows a close-up cross-sectional view of the armature conductors 11 before they are welded to the commutator 12. The commutator 12 includes a plurality of risers 30 defining slots 32. An insulator 34 separates each riser 30 of the commutator 12. The armature conductors 11 are placed in the slots 32 of the risers 30 before welding. Typically, two conductors 11 are placed in each slot 32. To join the conductors 11 to the commutator 12, the tungsten electrode 22 of the hot staking machine 20 is positioned over a slot 32 and moved downward into contact with the conductors 11, applying force to the conductors. As force is applied to the conductors, electric current is also provided to the electrode 22, causing the electrode to become heated. The combination of the heated electrode and force from the electrode softens the copper armature conductors and causes them to deform, preferably into an egg-shape. After a period of time, current to the electrode is terminated and the electrode is removed. Thereafter, the copper conductors re-harden and form a bond with the walls of the riser. After the hot staking machine 20 welds one set of conductors 11 in a slot 32, the armature is rotated to allow the hot staking machine to weld the next set of conductors in the respective slot.
A problem exists with the above-described method of heat staking because it is difficult to keep the tungsten electrode at a constant temperature. The electrode typically becomes hotter with each successive weld, as the same current is provided to the electrode during each weld and not much time is provided for cooling between welds. After several welds, the very hot electrode can cause damage by penetrating too far into the slot of the commutator when it contacts a conductor and causing the conductor to completely deform and melt into a U-shape around the electrode. These welds are faulty and are not capable of conducting current within an operating armature. Thus, there is a need in the industry for some quality control mechanism to be applied to the process of hot staking armature conductors to commutators in starter motors.
Some prior art machines and related methods have attempted to control the process of hot staking armature conductors to the commutators. A few of these methods have involved monitoring the displacement of the electrode in the slot when welding to keep the electrode from penetrating too far in the slot and resulting in a faulty weld. However, it has been noted that several of these prior art methods are overly complex and involve too many variables or are expensive to implement. For the foregoing reasons, there is a need in the industry for a relatively simple and inexpensive yet reliable method to be applied to the process of hot staking armature conductors to commutators that involves monitoring the displacement of the electrode in the slot of the commutator.
The present invention is directed to an apparatus and method that satisfies the need for a relatively simple, but reliable method for producing consistent weld qualities when hot staking armature conductors to commutators in starter motors. The apparatus includes a cradle used to rotatably support and mount a commutator. The commutator includes a plurality of slots and a plurality of conductors positioned in each slot. Once mounted in the cradle, the commutator is rotatable about the commutator axis such that one of the plurality of slots faces the upward direction. The apparatus also includes an electrode that is movable in the vertical direction to contact the exposed conductor in the upward facing slot of the commutator. A sensor determines how far the electrode has traveled within the slot. A compressor is provided to force the electrode against the armature conductor and compress the conductors in the upward facing slot of the commutator. A power supply is also provided for delivering current to the electrode.
The apparatus also includes a microprocessor in communication with other elements of the apparatus to control operation of the device. To this end, the microprocessor instructs the moveable electrode to contact the exposed conductor in the upward facing slot of the commutator and apply a force to the conductor. The microprocessor also instructs the power supply to deliver current to the electrode. As current is provided to the electrode in contact with the conductor, the conductor softens and begins to weld within the slot and the electrode moves further into the slot. When the electrode reaches a threshold displacement in the slot, the sensor sends a message to the microprocessor that the threshold displacement has been reached. The microprocessor then instructs the power supply to cease current delivery to the electrode after the electrode reaches the threshold displacement. After this, the electrode continues to apply force to the conductor until the microprocessor determines that a maximum weld time has been reached. Following the maximum weld time, the microprocessor instructs the compressor to remove the electrode from the armature conductor. If the maximum weld time is reached before the microprocessor receives the signal that the electrode has reached the threshold displacement, an error condition exists in the weld and an alarm is sounded.
Accordingly, the apparatus and method of the present invention provides a relatively simple, inexpensive and reliable method for producing consistent weld qualities when hot staking armature conductors to commutators in starter motors. These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.